1. Field of the Invention
The present invention relates to a process for the production of a pressure transducer or sensor using silicon on insulator technology, as well as the transducer or sensor obtained.
This transducer is usable in all fields where it is wished to measure a pressure and in particular in industrial fields such as the space and aeronautical fields, in the research field and also in connection with motor vehicles.
2. Brief Description of Related Prior Art
Numerous methods have been proposed for producing pressure transducers or micromachined silicon mechanical structures for transducers using microelectronic technologies.
The main advantage of silicon is clearly the collective treatment of the structures and their miniaturizations, i.e. a relatively low cost, as well as the mechanical reliability of the monocrystalline material used, which does not suffer from creep, hysteresis or time drift.
However, for an even more widespread use of these transducers, it is necessary to reduce even further the unit cost of the chip by reducing its surface, whilst retaining acceptable metrological qualities.
The standard technologies for the production of pressure transducers use volume anisotropic chemical machining of silicon, i.e. the entire substrate thickness is etched in order to free a sensitive monocrystalline structure.
The major disadvantages of this volume method are the use of a double face method (few existing specific machines, which are expensive and substrates polished on two faces); a shape of the transducer linked with the crystalline orientation of the substrate and therefore a limitation of these shapes; miniaturization of the transducer limited by the thickness of the substrate (three-dimensional structure, one dimension being fixed) and the need to carry out a sealing or bonding of the transducer on one or more substrates requiring the use of structure support and reference cavities, which somewhat complicates the manufacture of the transducers.
This volume method is described in reference 1--Transducers' 91 Digest of Technical papers, San Francisco, "Stability and common mode sensitivity of piezoresistive silicon pressure sensors made by different mounting methods" by R. Holm et al, pp. 978-981.
In general terms, the basic principle used in a silicon pressure transducer is the measurement of the deformation of a deformable silicon diaphragm or membrane mechanically connected to a support under the action of a pressure variation.
In order to produce pressure transducers in accordance with a volume method, it is necessary to have a selective layer during the etching of the substrate making it possible to bring about an etch stop at a clearly controlled depth. In addition, for mechanical strength reasons and also due to the electrical characteristics, it is very important for the freed structure to be of monocrystalline silicon.
The etch stop methods used are either the etching of a solid silicon substrate by the rear face with stopping on an epitaxial or epitaxially deposited silicon layer highly doped with boron (cf. reference 2--J.
Electrochem. Soc., Vol. 137, No. 11, November 1990, "Anisotropic etching of crystalline silicon in alkaline solutions : II. Influence of dopants" by H. Seidel et al. pp. 3626-3632), or an electrochemical etching of the silicon substrate with etch stop on an epitaxial or epitaxially deposited silicon layer forming a P/N junction with the substrate (cf. reference 3--IEEE Transactions on Electron Devices, Vol. 36, No. 4, April 1989, "Study of electrochemical etch-stop for high-precision thickness control of silicon membranes" by B. Kloeck et al., pp. 663-669).
These two etch stop methods suffer from the disadvantages referred to hereinbefore. Thus, they use an anisotropic etching of the substrate, limiting the shapes of the sensitive elements as a result of the crystalline orientation of the substrate, as well as etching by the rear face requiring the use of special substrates and a double face alignment procedure.
These stop methods also require the use of very selective etching masks and, taking account of the inclined etching planes (54.7.degree. for orientation 100 silicon) and the thickness of the silicon to be etched, the shapes produced on the rear face greatly exceed the final useful shapes of the component. The use of these very selective etching masks is in particular linked with a significant thickness of the silicon substrate to be etched.
When the sensitive element is finished, it is then necessary to bond it on one or more thick, rigid supports in order to obtain a transducer. This support or these supports generally have a different nature to the substrate (e.g. glass), which leads to differential stresses prejudicial to the performance characteristics of the transducer and also to a difficult, supplementary stage to be carried out.
Another way of producing pressure transducers consists of using a surface machining with the aid of a sacrificial layer and a deposited polycrystalline silicon layer producing the desired mechanical structure. This is described in reference 4--Transducer's 87, Tokyo, June 1987, "Fine grained polysilicon and its application to planar pressure transducers" by H. Guckel et al. pp. 277-282.
This method makes it possible to produce pressure transducers in single face technology by the use of the sacrificial layer. This surface machining has the major advantage of associating a simple method with structures having very small dimensions.
Unfortunately, this surface method suffers from disadvantages. In particular, the mechanical qualities of the material constituting the diaphragm (polycrystalline silicon or silicon nitride) are mediocre and significant differential thermal stresses are induced as a result of using different materials for the diaphragm and the substrate. This leads to pressure transducers having limited or inadequate metrological qualities.
Moreover, due to their nature, these deposits have a thickness limited to a few micrometres (generally below 2 .mu.m), which reduces the shape and size possibilities and prevents the use of a piezoresistive detection of the pressure.
In order to obviate these different disadvantages, the invention proposes a novel process for the production of a pressure transducer using silicon on insulator technology combined with a micromachining of the surface.
Silicon on insulator technology is known under the abbreviation SOI. One of the known methods makes use of recrystallization by laser of an amorphous or polycrystalline silicon layer deposited on a silicon dioxide layer obtained by the thermal oxidation of a monocrystalline silicon substrate. A second method known under the abbreviation SDB consists of carrying out the bonding of the two silicon substrates, whereof at least one has on the bonding surface a SiO.sub.2 layer, e.g. obtained by thermal oxidation, followed by the thickness reduction of one of the two substrates until the desired thickness is obtained (cf. Technical Digest MNE'90, 2nd Workshop, Berlin, November 90, pp. 81-86 by C. Harendt et al. "Wafer bonding and its application silicon-on-insulator fabrication".
A third known method is based on the implantation of oxygen or nitrogen ions with a high dose in solid monocrystalline silicon which, following annealing of the substrate at a high temperature, leads to the formation of a buried silicon oxide or silicon nitride insulating layer supporting a monocrystalline silicon film. The oxygen ion implantation technology is known as SIMOX technology. The invention makes more particular use of the third method.
WO91/19177 also describes a transducer having a deformable diaphragm and which can function by the capacitive effect. Thus, electrodes are formed on the substrate prior to producing the silicon and insulating layers, in the form of a p/n junction. However, such a process is not compatible with the SIMOX technology used in the present invention. SIMOX technology makes it necessary to heat the device to a high temperature close to 1350.degree. C. for about 6 hours in order to form the insulating layer and to restore the crystalline quality deteriorated during implantation. Under these conditions, any prior production by doping, such as the formation of an insulated electrode, with the aid of a p/n junction, is destroyed or deteriorated by the diffusion of species.